Residual Check Valve
Many master cylinder's designed for drum brakes incorporate a residual check valve to keep a slight amount of pressure on the system after the brake pedal has been released. These residual check valves are used on drum brake systems to keep around 5psi. up to 12psi. of pressure on the wheel cylinder's cup seals and resist any momentum of brake fluid returning to the master cylinder.
Since the valve is keeping constant pressure on the system, any small leaks will result in fluid leaking out instead of air leaking in. Residual check valves keeps a constant pressure on the wheel cylinder's cup seals.
These valves are located inside the master cylinder port leading to the rear drum brakes. They consist of a check valve and spring that allow brake fluid to enter the master cylinder only until a designated pressure is reached. Once this occurs, spring pressure closes the valve maintaining low pressure on the drum brakes. Many vehicle's today use cup expanders in the wheel cylinders that keep an air tight seal. This prevents air contamination eliminating the need for residual check valves.